1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus and a radio frequency (RF) coil, and especially to decoupling of an array coil that is formed by coupling multiple coil elements.
2. Description of the Related Art
A conventional magnetic resonance imaging apparatus is a device that creates an image of the inside of the body of a subject by use of a magnetic resonance phenomenon, and the apparatus includes an RF coil to detect a nuclear magnetic resonance signal generated from the subject. An “array coil” has been widely adopted for the RF coil. An array coil is formed by coupling multiple coil elements so that the apparatus can detect signals from a wide range of area that cannot be covered by a single coil element (see JP-A No. 2007-21188 (KOKAI), for example).
More specifically, the array coil is formed of a coil element group that includes multiple coil elements. Loop coils and figure-eight coils (also referred to as saddle coils) may be used for the coil elements, and various arrangement patterns are conceivable for the coil elements. FIGS. 14A to 14D are diagrams for explaining geometrical arrangements of coil element groups in conventional array coils.
In FIG. 14A, an example arrangement incorporating three loop coils is illustrated. With this arrangement, a high sensitivity of the coil element group can be achieved deep in the depth direction and wide in the lateral direction of the subject. The signal to noise ratio (S/N) is at a standard level. In addition, an example arrangement incorporating a loop coil and a figure-eight coil is illustrated in FIG. 14B. With this arrangement, a high sensitivity of the coil element group can be achieved deep in the depth direction but narrow in the lateral direction of the subject. The S/N is high at the center of the lateral direction.
Furthermore, an example arrangement incorporating four loop coils is illustrated in FIG. 14C. With this arrangement, a high sensitivity of the coil element group can be achieved shallow in the depth direction but wide in the lateral direction of the subject. The S/N is at a standard level. In FIG. 14D, an example arrangement incorporating three loop coils and one figure-eight coil is illustrated. With this arrangement, a high sensitivity of the coil element group can be achieved deep in the depth direction and wide in the lateral direction of the subject. The S/N is at a standard level on the left and right sides of the lateral direction, and excellent at the center thereof.
When combining several coil elements as described above, the sensitivity and the S/N of the coil element group vary, depending on what types of coil elements are combined and how they are arranged. For this reason, when preparing an array coil specialized for a certain imaging area, a coil element group suitable for the area should be selected.
For example, among the coil element groups illustrated in FIGS. 14A to 14D, a coil element group that can be placed on the back of the subject to take an abdominal image and also a spinal image will be considered. First, because a high detection sensitivity is required deep in the depth direction and wide in the lateral direction of the subject when taking an abdominal image, the coil element groups illustrated in FIGS. 14A and 14C are the most suitable. On the other hand, because a high S/N is required at the center of the lateral direction of the subject when taking a spinal image, the coil element group illustrated in FIG. 14B is the most suitable.
Thus, the coil element group of FIG. 14D that has the structures of both FIGS. 14A and 14B is conceivable as the most suitable coil element group for taking an abdominal image and also a spinal image.
When the coil element group of FIG. 14D is placed on the back of the subject for taking an abdominal image and also for taking a spinal image, however, it is difficult to decouple the coil elements.
In particular, the adjacent loop coils can be easily decoupled by overlaying part of the loop coils and adjusting the area of the overlapping portion. Furthermore, the center loop coil and the figure-eight coil can be structurally decoupled because the sum of the high-frequency magnetic fields generated by the loop coil and by the figure-eight coil is zero.
It is difficult, however, to decouple the loop coils at the two ends and the figure-eight coil. Even if part of the loop coils and part of the figure-eight coil are overlaid, the lateral range in which the loop coils are allowed to be arranged is limited due to the structure of the apparatus, and therefore the area of the overlapping portion cannot be easily adjusted.